The proposed project will take advantage of the unique opportunity to investigate adolescents prior to or at the early stages of substance use to identify brain-based biomarkers associated with initiation of use, the severity of use and the trajectory of use over time, and the impact of use on the developing brain. The existing, cross- sectional and longitudinal data set (R01AA016274 and ROIMH87493) includes behavioral measures and neuroimaging data for 330 adolescents (age 12-15) at baseline and 264 (age 14-17) at two-year follow-up. Preliminary analyses of ASL-MRI images at baseline using a general index of substance use severity (report of trying two or more substances, SU2+) indicate that resting regional cerebral blood flow to cortico-limbic brain regions is elevated in SU2+ adolescents (n=21) relative to those who were abstinent (n=98). The work proposed will (1) include more qualified severity metrics to clarify associations with rCBF, (2) determine whether baseline cortico-limbic rCBF is associated with future substance use over the two-year clinical follow- up, and (3) the effects of chronic substance use on the developing brain. In addition to the uniqueness and richness of the sample, the focus on resting rCBF is ideal for studying the developing brain as blood flow is required in brain regions where angiogenesis, synaptogenesis or other processes of neurodevelopment are occurring. However, to date quantitative measure of regional cerebral blood flow (rCBF) has been restricted to adult populations. As well, neuroimaging studies of substance use have been limited to adult, chronic users. Therefore, the outcomes produced from this study will be the first reported in this age group, particularly as rCBF relates to substance use.